1. Field of the Invention
The present invention relates to a current mirror circuit composed of n-channel field effect transistors, and reference voltage generating and light emitting element driving circuits using this current mirror circuit.
2. Related Background Art
In an integrated circuit mainly composed of bipolar transistors such as those of ECL (Emitter Coupled Logic), the current flowing through the circuit is substantially determined by the base potential of the transistor connected to its lower-potential side power supply V.sub.SS. This potential is generated by a reference voltage generating circuit, which is typically based on a current mirror circuit. Namely, as shown in FIG. 6, its higher-potential side power supply V.sub.DD is provided with a first current mirror circuit composed of a pair of PNP transistors Q1 and Q2, whereas the lower-potential side power supply V.sub.SS is provided with a second current mirror circuit composed of a pair of NPN transistors Q3 and Q4, in order to determine the base potential V.sub.B of the second current mirror circuit, i.e., reference potential V.sub.REF, in response to the current I.sub.C determined by the first current mirror circuit.
In order for a circuit equivalent to that of FIG. 6 to be composed of field effect transistors (FETs), it has conventionally been necessary for a p-channel FET (p-FET) to be used in the first current circuit disposed on the side of the higher-potential side power supply V.sub.DD.
The p-FET, however, has been disadvantageous in that it has inferior high-frequency characteristics and lower gain as compared with an n-channel field effect transistor (n-FET). Consequently, in order to secure a necessary current, a wider device area is required, thus making it difficult to improve the packaging density when n-FETs are integrated into a circuit, for example. Further, no p-channel Schottky field effect transistor (MESFET) has been realized yet, since it has not been able to raise the Schottky barrier between metals and p-type semiconductors.
In view of these problems, it is an object of the present invention to provide a current mirror circuit which is composed of n-channel field effect transistors and which can be connected to the side of a higher-potential side power supply. It is another object of the present invention to provide reference voltage generating and light emitting element driving circuits using this current mirror circuit.